Ion implant monitoring through measurement of modulated optical response

ABSTRACT

A method for simultaneously monitoring ion implantation dose, damage and/or dopant depth profiles in ion-implanted semiconductors includes a calibration step where the photo-modulated reflectance of a known damage profile is identified in I-Q space. In a following measurement step, the photo-modulated reflectance of a subject is empirically measured to obtain in-phase and quadrature values. The in-phase and quadrature values are then compared, in I-Q space, to the known damage profile to characterize the damage profile of the subject.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.10/387,259, filed Mar. 12, 2003, which claims priority from U.S.Provisional Application Ser. No. 60/365,237, filed Mar. 18, 2002, andfrom U.S. Provisional Application Ser. No. 60/378,140, filed May 14,2002, each of which is hereby incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The subject invention relates to optical devices used tonon-destructively evaluate semiconductor wafers. In particular, thepresent invention relates to systems for measuring dopant concentrationsin semiconductor samples.

BACKGROUND

As geometries continue to shrink, manufacturers have increasingly turnedto optical techniques to perform non-destructive inspection and analysisof semiconductor wafers. The basis for these techniques is the notionthat a subject may be examined by analyzing the reflected energy thatresults when an optical beam is directed at a subject. This type ofinspection and analysis is known as optical metrology and is performedusing a range of different optical techniques.

One widely used type of optical metrology system, as shown in FIG. 1,includes a pump laser. The pump laser is switched on and off to createan intensity-modulated pump beam. The pump beam is projected against thesurface of a subject causing localized heating of the subject. As thepump laser is modulated, the localized heating (and subsequent cooling)creates a train of thermal and plasma waves within the subject. Thesewaves reflect and scatter off various features and interact with variousregions within the sample in a way that alters the flow of heat and/orplasma from the pump beam spot.

The presence of the thermal and plasma waves has a direct effect on thesurface reflectivity of the sample. Features and regions below thesample surface that alter the passage of the thermal and plasma waveswill therefore alter the optical reflective patterns at the surface ofthe sample. By monitoring the changes in reflectivity of the sample atthe surface, information about characteristics below the surface can beinvestigated.

To monitor the surface changes, a probe beam is directed at a portion ofthe subject that is illuminated by the pump laser. A photodetectorrecords the intensity of the reflected probe beam. The output signalfrom the photodetector is filtered to isolate the changes that aresynchronous with the pump beam modulation. For most implementations,this is performed using a heterodyne or lock-in detector (See U.S. Pat.No. 5,978,074 and in particular FIG. 2 for a discussion of such alock-in amplifier/detector). Devices of this type typically generateseparate “in-phase” (I) and “quadrature” (Q) outputs. These outputs arethen used to calculate amplitude and phase of the modulated signal usingthe following equations:Amplitude={square root}{square root over (I ² +Q ²)}  (1)Phase=arctan(Q/I)  (2)

The amplitude and phase values are used to deduce physicalcharacteristics of the sample. In most cases, this is done by measuringamplitude values (amplitude is used more commonly than phase) for one ormore specially prepared calibration samples, each of which has knownphysical characteristics. The empirically derived values are used toassociate known physical characteristics with corresponding amplitudevalues. Amplitude values obtained for test subjects can then be analyzedby comparison to the amplitude values obtained for the calibrationsamples.

Systems of the type shown in FIG. 1 (i.e., those using external means toinduce thermal or plasma waves in the subject under study) are generallyreferred to as PMR (photomodulated reflectance) type systems. PMR-typesystems are used to study a range of attributes, including materialcomposition and layer thickness. PMR-type systems and their associateduses are described in more detail in U.S. patent Ser. Nos.: U.S. Pat.Nos. 4,634,290; 4,646,088; 4,679,946; 4,854,710; 5,854,719; 5,978,074;5,074,669; and 6,452,685. Each of these patents is incorporate in thisdocument by reference.

Another important use of PMR-type systems is measurement and analysis ofthe dopants added to semiconductor wafers. Dopants are ions that areimplanted to semiconductors during a process known as ion implantation.The duration of the ion implantation process (i.e., total exposure ofthe semiconductor wafer) controls the resulting dopant concentration.The ion energy used during the implantation process controls the depthof implant. Both concentration and depth are critical factors thatdetermine the overall effectiveness of the ion implantation process.

PMR-type systems are typically used to inspect wafers at the completionof the ion implantation process. The ion implantation damages thecrystal lattice as incoming ions come to rest. This damage is typicallyproportional to the concentration and depth of ions within the crystallattice. This makes measurement of damage an effective surrogate fordirect measurement of dopant concentration and depth. PMR-type systemshave proven to be adept at measuring damage and have been widely usedfor post implantation evaluation.

As shown in FIG. 2, the relationship between dopant concentration andamplitude measurements (i.e., as defined by Equation (1)) is monotonicfor low dopant concentrations. As dopant concentrations increase (e.g.,greater than 1E14 for As⁺ or P⁺ ions or greater than 1E15 for B⁺ ions)the monotonic relationship breaks down. In fact, at high concentrations,the amplitude measurements are no longer well behaved and as a resultcannot be used to accurately derive corresponding dopant concentrations.In FIG. 1, this is illustrated by the points A, B and C all having thesame the same amplitude measurement, even though each point represents adifferent dopant concentration. The same sort of breakdown occurs as thetype of implanted ions becomes heavier (e.g., As⁺ or P⁺ ions). In bothcases, this is attributable to the appearance of a Si amorphous layerresulting in optical interference effects. Although not shown in FIG. 2,phase information becomes flat or insensitive to changes inconcentration at high dopant concentrations or where heavy ions areimplanted.

One approach for dealing with the problem of monitoring samples withhigh dopant concentrations is to measure the DC reflectivity of both thepump and probe beams in addition to the modulated optical reflectivitysignal carried on the probe beam. Using the DC reflectivity data at twowavelengths, some ambiguities in the measurement can often be resolved.The details of this approach are described in U.S. Pat. No. 5,074,669(incorporated in this document by reference).

In general, PMR-type systems of the type described above have proven tobe effective methods for testing and characterizing semiconductordevices. Their ability to function in a non-contact, non-destructivefashion, combined with their high-accuracy and repeatability haveensured their widespread use as part of semiconductor manufacturing.Still, there is an obvious need for methods to provide this type ofmeasurement capability for high dopant concentrations and ionimplantation of relatively heavy ions.

BRIEF SUMMARY

The present invention provides a method of simultaneously monitoring ionimplantation dose, damage and/or dopant depth profiles in ion-implantedsemiconductors. For this method, a PMR-type optical metrology tool isused to record both quadrature (Q) and in-phase (I) values for a seriesof specially prepared calibration subjects. Each calibration subject isfabricated at the same implantation energy. As a result, variationsrecorded by the PMR-type system are largely attributable to variationsin dopant concentration.

The measurement method performs a linear fit using the recorded pointsto define a calibration line within an I-Q plane. The slope of thecalibration line is defined by the implantation energy used to createthe calibration subject. Points along the calibration line correspond todifferent dopant concentrations. The calibration line is used to definea calibration region within the I-Q plane. The calibration regionincludes all points within a specified distance (often defined in termsof a percentage) of the calibration line. Typically, this is done bydefining an upper boundary line that has a slightly greater slope thanthe calibration line and a lower boundary line that has a slightlysmaller slope than the calibration line. The calibration region is thearea between the upper and lower boundary lines.

After creating the calibration region, the PMR-type optical system maybe used to inspect and analyze semiconductor wafers. For each subjectwafer, the PMR-type system makes one or more measurements. Measurementsthat fall within the calibration region are known to share the damageprofile of the calibration subject. Measurements that do not fall withinthis region are assumed to deviate from the known damage profile of thecalibration subject. This test provides an effective method of acceptingor rejecting wafers that provide acceptable accuracy even when dopantconcentrations are high or where heavy ions have been implanted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a photo-modulated reflectometer as used by thepresent invention.

FIG. 2 is a plot of amplitude measurements as a function of implantdose, from the sample output of the photo-modulated reflectometer ofFIG. 1.

FIG. 3 is a plot of quadrature (Q) and in-phase (I) values recorded bythe photo-modulated reflectometer of FIG. 1 for a series of subjectswithin a specially prepared calibration set.

FIG. 4 shows a calibration line that best fits the I-Q points of FIG. 3.

FIG. 5 shows a calibration region centered around the calibration lineof FIG. 4.

FIG. 6 shows the use of the calibration region of FIG. 5 to accept orreject measurements recorded by the photo-modulated reflectometer ofFIG. 1.

FIG. 7 is a second plot of quadrature (Q) and in-phase (I) valuesrecorded by the photo-modulated reflectometer of FIG. 1 for a series oflocations within a specially prepared calibration subject.

FIG. 8 shows a series of calibration lines that best fit the I-Q pointsof FIG. 7.

DETAILED DESCRIPTION

The present invention provides a method for simultaneously measuring ionimplantation dose, damage and/or dopant depth profiles in ion-implantedsemiconductors. The measurement method is logically divided into twosteps: a calibration and a measurement step. During the calibrationstep, the photo-modulated reflectance of a known damage profile ischaracterized. Typically, this involves identifying one or more areaswithin I-Q space that correspond to the photo-modulated reflectance ofthe known damage profile. All other areas within I-Q space are thenassumed to be dissimilar to the known damage profile. In the measurementstep, I-Q measurements for a test subject are obtained empirically. Theempirically obtained I-Q measurements are then compared to determine ifthey fall within an identified region of I-Q space. This comparisonindicates whether the test subject has a damage profile that is similarto the known damage profile. The following sections describe severalpossible implementations for both the calibration and measurement steps.

For a first implementation of the calibration step, a PMR-type opticalmetrology tool is used to record both quadrature (Q) and in-phase (I)values for a series of specially prepared calibration subjects. Eachcalibration subject is fabricated at the same implantation energy. As aresult, variations recorded by the PMR-type system are largelyattributable to variations in dopant concentration. Each measured valueis treated as a point within an I-Q plane. FIG. 3 shows a representativeseries of measured values plotted as points within an I-Q plane. Thecalibration step uses a linear fitting algorithm (such as least squares)to define a calibration line that best fits the points within the I-Qplane. FIG. 4 shows a calibration line that corresponds to therepresentative points of FIG. 3. The slope of the calibration line isdefined by the implantation energy used to create the calibrationsubjects. Points along the calibration line correspond to differentdopant concentrations.

The calibration line is used to define a calibration region within theI-Q plane. The calibration region includes all points within a specifieddistance (often defined in terms of a percentage) of the calibrationline. As shown in FIG. 5, this is typically accomplished by defining anupper boundary line and a lower boundary line. The upper boundary linehas a greater slope and the same Q-intercept as the calibration line.The lower boundary line has a smaller slope and the same Q-intercept asthe calibration line. The calibration region is the area within the Iand Q space that is bounded by the upper and lower boundary lines.

For the associated measurement step, the PMR-type optical system istypically used to inspect and analyze a series of semiconductor wafers.For each subject wafer, the PMR-type system makes one or moremeasurements. Each measurement includes both I and Q values and definesa point within the I-Q plane. For the measurement method, the proximityof each point to the calibration line measures the similarity of thatpoint to the damage profile of the calibration subject. Points that areclose to the calibration line represent minor departures from the dopantdepth and concentrations of the calibration subjects. Points that arefurther away represent larger departures. Points that fall outside ofthe calibration region (shown as black dots in FIG. 6) represent evenlarger departures from the calibration subject. These points are assumedto represent large deviations from the known damage profile of thecalibration subject resulting from channeling effects, wafer/beamnonuniformities, etc. It should be noted that points that inside of thecalibration region or even on the calibration line (such as point X) canbe very similar in amplitude to points that fall outside of thecalibration regions (such as point X¹). In a prior art system, thatexamines only amplitude, the difference between these two points wouldbe undetectable. Similar ambiguities can also arise where points thathave different amplitudes correspond, in fact, to the same damageprofile. Prior art techniques would be unable to detect the similarityof such points.

For a second implementation of the calibration step, a PMR-type opticalmetrology tool is again used to record both quadrature (Q) and in-phase(I) values for a series of specially prepared calibration subjects. Arepresentative series of points of this type are shown in FIG. 7. Asshown in FIG. 8, the calibration step uses these points to create aseries of calibration lines. Each line is localized to fit a subset ofthe points measured by the PMR-type optical metrology tool. In thisexample, three calibration lines have been defined. The first islocalized to fit the points that are local to point A. The second andthird are localized to fit the points near point B and C, respectively.The slope of each calibration line reflects its associated implantationenergy. Points along each line reflect different dopant concentrations.

For the associated measurement step, each point measured by the PMR-typeoptical metrology tool is compared to see if it lies on or near any ofthe calibration lines. Points on or nearby calibration lines share thedamage profile of the associated calibration line. Points that are notnear (or on) calibration lines represent major departures from thedopant depth and concentration of the calibration subjects due tochanneling effects, wafer/beam nonuniformities, etc. It should be notedthat points on different calibration lines (such as points A, B and C)can have identical amplitudes. Using prior art techniques that examineonly amplitude, the difference between these points would beundetectable. Similar ambiguities can also arise where points that havedifferent amplitudes correspond, in fact, to the same damage profile.Prior art techniques are unable to detect the similarity of such points.

The preceding description has focused on the use of in-phase (I) andquadrature (Q) signals. It is important to realize that there may beimplementations that use linear combinations of these signals, in placeof the I and Q values. This description and the following claims arespecifically intended to cover all useful linear combinations of thistype, without limitation.

It should be noted that this approach is useful in systems that measurethe modulated reflectivity of the probe as well as systems that monitorother periodic surface variations such as in interferometry systems orperiodic angular variations (“pump” type systems). To the extent theseexperiments are performed on semiconductor samples, it should also beunderstood that a portion of the signal would be the result of themodulated electron hole plasma as opposed to being a purely thermalsignal. The relative contributions of the plasma and thermal effects onthe signals depends on the dosage level and experimental conditions suchas pump and probe beam wavelengths, beam spot size and pump modulationfrequency.

It should also be noted that the measurement method is useful both asdescribed, and as part of a more complex analysis. This means, forexample that there may be cases where the measurement method will beused in combination with related measurements that analyze either orboth of amplitude and phase information.

1. A method of optically inspecting and evaluating ion implantation in asubject, the method comprising: identifying calibration points within anI-Q space that correspond to the photo-modulated reflectance of knownion concentrations; making multiple photo-modulated reflectancemeasurements of the subject to obtain in-phase and quadrature values;and comparing the in-phase and quadrature values obtained from thesubject to the calibration points to compare the ion concentrations ofthe subject to the known ion concentrations.
 2. A method as recited inclaim 1, wherein: at least some of the calibration points are identifiedempirically by analyzing the photo-modulated reflectance of acalibration subject.
 3. A method as recited in claim 1, wherein: each ofthe multiple photo-modulated reflectance measurements includes the stepsof: periodically exciting a region on the sample; directing a probe beamto reflect off the region on the sample surface that has beenperiodically excited; monitoring the reflected probe beam and generatingan output signal in response thereto; and analyzing the output signalwith a phase synchronous detection system and generating in-phase andquadrature signals
 4. A method as recited in claim 1, wherein: at leastsome of the calibration points are identified by extrapolating fromempirically identified points.
 5. A method as recited in claim 4,wherein: the process of extrapolation includes defining one or morecalibration lines within I-Q space, each calibration line being a linearfit of a set of empirically identified points.
 6. A method of evaluatingion concentration in a sample comprising the steps of: periodicallyexciting at least one region on the sample; directing a probe beam toreflect off each region on the sample surface that has been periodicallyexcited; monitoring the reflected probe beam at each region andgenerating an output signal in response thereto; analyzing the outputsignal with a phase synchronous detection system and generating in-phaseand quadrature signals; and evaluating the ion concentration of eachregion by comparing values of the in-phase versus the quadrature signalsto predetermined reference values.
 7. A method as recited in claim 6that further comprises: evaluating the sample by comparing the value ofeither the amplitude or phase of the signals to predetermined referencevalues.
 8. A method as recited in claim 6, wherein: the predeterminedreference values are identified empirically by analyzing thephoto-modulated reflectance of at least one calibration subject.
 9. Amethod as recited in claim 6, wherein: a linear combination of in-phaseand quadrature signals is evaluated.
 10. A device for evaluating ionconcentration in a sample, the device comprising: a first illuminationsource producing an intensity modulated beam for periodically excitingat least one region on the sample; a second illumination sourceproducing a probe beam to reflect off each region on the sample surfacethat has been periodically excited; a detector for monitoring thereflected probe beam at each region and generating an output signal inresponse thereto; a lock-in amplifier for analyzing the output signal togenerate in-phase and quadrature signals; and a processor for evaluatingion concentration at each region by comparing values of the in-phaseversus the quadrature signals to predetermined reference values.
 11. Adevice as recited in claim 10, wherein: the processor evaluates thesample by comparing the values of either the amplitude or phase of thesignal to predetermined reference values.
 12. A device as recited inclaim 10, wherein: the predetermined reference values are identifiedempirically by analyzing the photo-modulated reflectance of at least onecalibration subject.
 13. A device as recited in claim 10, wherein: theprocessor evaluates a linear combination of in-phase and quadraturesignals.
 14. A method of evaluating variations in dopant concentrationin a semiconductor sample, the method comprising: directing an intensitymodulated pump beam and a probe beam to the sample surface; obtainingmeasurements by analyzing the reflected probe beam, each measurementcomposed of an in-phase (I) value and a quadrature (Q) value; andanalyzing variations in dopant concentration for the semiconductorsample as a function of the I and Q values included in the measurements.15. A method as recited in claim 14, further comprising: comparing thevariations in dopant concentration to previously derived calibrationpoints associated with at least one calibration sample.
 16. A method asrecited in claim 14, wherein: the I and Q values are compared tocalibration I and Q values obtained from one or more calibration sampleshaving known dopant concentration values.